In drive systems of the past, the outside diameter of a worm gear was constrained to be equal to the shaft or bearing diameter for ease of assembling through a bearing for a vehicle window drive system. The worm gear typically was rotatively driven by a drive motor and was used to drive a driven gear. The torque capacity of the worm gear was much less than that obtainable with an enveloped worm gear. The torque capacity of a worm gear is limited by having a primary or single contact tooth which contacts a mating tooth of the driven gear. Prior known enveloped worm gears have required that the teeth be specially machined and therefore are of limited application in vehicles requiring mass produced parts using conventional manufacturing techniques. Prior attempts have been made to use a conventional roll manufacturing technique to generate a multi-contact worm gear; however, such designs produced contact points where the gear teeth of the drive gear contacted mating teeth of driven gear situated on an arc which was concentric about the axis of the driven gear. This had the disadvantage of causing the multiple teeth to contact the driven teeth at a single contact location on the driven gear resulting in increased wear at that contact point, thereby reducing the useful life of the driven gear.
If a drive gear is used in a situation requiring driven movement in both a counter clockwise and clockwise direction both sides of each tooth are used to drive the load. Often times, the torque or load experience by the gear is not the same in both rotational directions. For example, when a passenger is seated on a power seat of a vehicle, a maximum load is experienced when the passenger raises the seat, while a minimum load is experienced when the passenger lowers the seat.
Prior known systems have provided a worm gear having teeth with sides which define pressure angles that are the same on both sides. When such a worm gear is used in a multi-torque environment, the gear experiences a maximum bending stress which is not balanced, and requires the motor to be of a capacity which can handle the maximum bending stress associated with the largest load.